I realized today that I have an idling identical Proxmox box in my office where I can run the TrueNAS Scale Beta VM.
I was able to easily get the docker version of Nextcloud on there. I had read about issues with container versions not working with proxies, I still need to test that.
I'm running a Nextcloud Pi VM right now, but I don't really need any of the extra features it provides, and it doesn't get updates as quickly as the more direct versions, so the NCP version is still on Hub 9 where the Docker version on TrueNAS Scale is on Hub 10.